The Margin of error is the statistical way of calculating the random error in a given sample. It tells us to what extent the survey is reliable!. It is expressed in percentage.
Margin of Error formula is given by

Margin of Error

Where P is the probability of proportion of population,

N gives the size of the population,

n is the size of the sample.

Suppose you are having the set of children in which there are 60% of them participating in sports. If the marginal error is 5% it means that there is possibility of 55% - 65% people participating in sports. Thus the marginal error tells you how accurate the results are!. The greater the marginal error, lesser will be the accuracy. 

Marginal Error Calculator is a online tool that gives the marginal error of the given survey if the size of the population N, probability p and sample size n are entered in blocks provided.  
Here are given some steps to find the Margin of Error.

Step 1 : Read the problem and observe the given quantities

Step 2 : The Margin of error formula is given by

MOE = (1.96) $\sqrt{\frac{N - n}{N - 1}}$ $\times$ $\sqrt{p \frac{1 - p}{n}}$

Substitute the values and get the answer.